ISTANBUL (AP) — U.S. Secretary of State John Kerry struggled Sunday to convince Turkey's leaders they should promptly restore full diplomatic ties with Israel, two American allies counted on by President Barack Obama to help calm the turbulent Middle East.

But Turkey demanded that Israel first end all commercial restrictions against the Palestinians before the once-close partners could end their estrangement, which stems from an Israeli raid in 2010 on a flotilla bound for the Gaza Strip. Eight Turks and a Turkish-American died.

Obama revived the rapprochement during a visit to Israel last month, and Kerry aimed to firm that up in Istanbul, the first stop in a 10-day trip.

The stakes are high, given that the U.S. sees Turkey and Israel as anchors of stability in a region riven by Syria's civil war, Arab Spring political upheavals and the potential threat posed by Iran's nuclear program.

"We would like to see this relationship that is important to stability in the Middle East and critical to the peace process ... get back on track in its full measure," Kerry told reporters at a news conference with Foreign Minister Ahmet Davutoglu.

Kerry said that meant promises of "compensation be fulfilled, ambassadors be returned and that full relationship be embraced."

US Secretary of State John Kerry speaks during a news conference with his Turkish counterpart Ahmet …

He also met with Prime Minister Recep Tayyip Erdogan and then went to Israel.

Obama, before leaving Israel two weeks ago, arranged a telephone conversation between Israeli Prime Minister Benjamin Netanyahu and Erdogan. Netanyahu apologized for the flotilla incident; compensation talks are expected to begin this week.

But Davutoglu suggested that full normalization of ties would probably take some time.

"There is an offense that has been committed and there needs to be accountability," Davutoglu said.

He signaled that Turkey would pursue a "careful" advance toward a complete restoration of relations, with compensation and an end to Israeli trade restrictions on the Gaza Strip as the stumbling blocks.

US Secretary of State John Kerry, left, shake hands with his Turkish counterpart Ahmet Davutoglu aft …

More than 70,000 people have died in the war, according to the United Nations, but the U.S. fears it could get even worse, by spilling into neighboring countries or through the use of chemical weapons.

Both potential scenarios have led to intense contingency planning among Washington and its regional partners, including Israel and Turkey.

Kerry, who noted his twice-weekly telephone chats with Davutoglu, spoke of shared U.S. and Turkish efforts to support Syria's opposition coalition.

The opposition has suffered from poor coordination between its political leadership and the military factions leading the fight against the government of Syrian President Bashar Assad, and from intense infighting among those who seek to guide the amorphous movement's overall strategy.

Turkey has gone further than the U.S. in its assistance, accepting some 180,000 Syrians as refugees and sending advanced weaponry to rebels fighting to overthrow Assad.

The U.S. is only providing nonlethal aid to the rebels in the form of meals, medical kits and training.

Kerry praised Turkey for its generosity toward refugees and commitment to keeping its borders open, an issue of growing U.S. concern as the outflow of Syrians stretches the capacities of neighboring countries to accommodate them.

"The United States and Turkey will continue cooperating toward the shared goal of a peaceful transition in Syria," he said.

After arriving in Israel for his trip there in two weeks, Kerry went directly to the West Bank city of Ramallah, where he met with Palestinian President Mahmoud Abbas.

He planned to see Netanyahu and other senior Israeli and Palestinian officials Monday and Tuesday as part of a fresh American bid to unlock the long-stalled Middle East peace process.

Palestinian President Mahmoud Abbas, right, meets with U.S. Secretary of State John Kerry in the Wes …

Conversations in Israel will also cover shared U.S. and Israeli concerns over Iran's nuclear program.

Representatives from the U.S. and other world powers met the Islamic republic in Kazakhstan for another round of negotiations, but no breakthrough was announced on a proposed deal that would see international penalties eased on Iran if Tehran convinces the world that it is not trying to develop nuclear weapons.

Kerry said the "door is still open" for a negotiated agreement, but that the onus was on the Iranians.

"If you have a peaceful program for nuclear power, as a number of nations do, it's not hard to prove that," he said. "They have chosen not to live up to the international requirements and standards with respect to verification of their program."

The other stops on his trip are Britain, South Korea, China and Japan. He returns to Washington on April 15.